Extended Example (Iterative Refinement) A maze consists of rooms and - - PowerPoint PPT Presentation

Extended Example (Iterative Refinement)

A maze consists of rooms and doors:

• An door is either

a door into a room an escape to a particular place

• A room has two doors, left and right

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Door Data Definition

abstract class Door { } class Into extends Door { Room next; Into(Room next) { this.next = next; } } class Escape extends Door { String name; Escape(String name) { this.name = name; } }

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Room Data Definition

class Room { Door left; Door right; Room(Door left, Door right) { this.left = left; this.right = right; } } Copy

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Factory for Examples

class Factory { Factory() { } Room Example() { Door meadow = new Escape("meadow"); Door street = new Escape("street"); Room ms = new Room(meadow, street); Room planets = new Room(new Escape("mars"), new Escape("venus")); return new Room(new Into(ms), new Into(planets)); } }

Copy Local definitions ⇒ Intermediate Java

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Finding Paths

• Implement the Door method canEscape that takes a string and returns

a boolean indicating whether an escape with the given name is available

• Replace the canEscape method with a escapePath method that takes

a string and returns either a path of "left" and "right" leading to the escape,

• r a failure value

Path escapePath(String dest)

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Paths

A path result is either

• failure
• immediate success
• left followed by a (succesful) path
• right followed by a (successful) path

We'll need a Path abstract class with an isOk method

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Paths

abstract class Path { abstract boolean isOk(); } class Fail extends Path { Fail() { } boolean isOk() { return false; } } class Success extends Path { Success() { } boolean isOk() { return true; } } class Right extends Path { Path rest; Right(Path rest) { this.rest = rest; } boolean isOk() { return true; } } class Left extends Path { Path rest; Left(Path rest) { this.rest = rest; } boolean isOk() { return true; } }

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Door Variations and Person Attributes

Eventually, we want locked doors, short doors, magic doors, and

• ther kinds of doors

Finding an escape will depend on having keys, being a certain height, etc. Instead of adding more and more arguments to escapePath, let's introduce a Person to carry attributes

• Replace the destination-string argument of escapePath with a Person

argument, where a Person has a destination and height

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Short Doors

• Add a new kind of door, a short door, where a person must be less that the

door's height to pass Adding a short door requires only the declaration of a Short class — no other code changes!

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Locked Doors

• Add a new kind of door, a locked door, where a person must have a key to

pass Besides adding Locked, we change Person to add the notion of keys to the person In contrast to adding new variants, adding new operations requires changing the class

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Scheme versus Java

Scheme: New variant ⇒ change old functions New function ⇒ no changes to old code Java: New variant ⇒ no changes to old code New method ⇒ change old classes This is the essential difference between functional programming and

• bject-oriented programming

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